The present invention relates to a manufacturing apparatus of a disc substrate by means of injection molding, and in particular, relates to a mold used in the manufacturing apparatus.
One method of producing a disc substrate is known as an injection molding method of resin such as polycarbonate. An injection molding apparatus for this method has fixed type and movable type of molds arranged face to face. In both molds, two mirror plates facing with each other are placed and a gap is formed when those plates are butted. A stamper on which desired information tracks are formed is disposed on the fixed type of mold, in which a cavity formed by the mirror plates is filled with molten resin. A cooling water ditch is formed on each mirror plate in order to cool the resin filled in the cavity. When the resin has been cooled down to an adequate temperature and set, the movable type of mold is retreated to take out a disc made of the set resin. Such a disc manufacturing method is exemplified by a Japanese Patent Laid-open (KOKAI) Publication No. 11-192642.
As described above, in the injection molding apparatus for disc substrates, for the purpose of cooling resin between the molds, the cooling water ditch is formed on the back of the mirror plates of the molds.
FIG. 1 is a plan view showing one example of the shape of such a cooling water ditch. Usually, a cooling water ditch is formed on a mirror plate 30 of a specified thickness. In FIG. 1, the cooling water ditch 31 includes a plurality of circumferential parts 32 shaped into concentric circles and a plurality of straight parts 34 connecting those circumferential parts 32 to each other. Cooling water enters this cooling water ditch from, for example, an innermost inlet 38 thereof, flows along its ditch path consisting of the circumferential parts 32 and straight parts 34, and flows out of an outlet 40. Hereinafter, an area in which the straight parts 34 are arranged in the radius direction is also called flex section 36 (refer to a dotted area in FIG. 1).
In general, when resin is injected in a cavity formed between the mirror plates of the mold, the mirror plates are brought into distortion in the shape of a bowl because of injection pressure caused by the injected resin. In other words, the two facing mirror plates, in themselves, deflect due to the injection pressure, with the result that a certain amount of deflection caused in the central area is slightly larger than that in the peripheral area thereof.
However, as shown in FIG. 1, in cases the cooling water ditch includes the flex section 36, an amount of distortion in the flex section 36 becomes uneven due to the injection pressure compared to that in the remaining section. Hence, disc substrates that are manufactured using a mold having such mirror plates have uneven thicknesses. It is a general tendency that the flex section 36 or thereabout of a disc substrate to be manufactured is thinner in the circumferential direction than the remaining section thereof. Therefore, the aberration of a light beam reading information becomes larger in reproducing information from the disc, thus leading to problems, such as deterioration of signals and others.
The present invention has been made with consideration of the foregoing inconveniences, and an object of the present invention is to provide a disc-manufacturing mold capable of manufacturing a disc substrate with almost no irregularities in thickness in the circumferential direction and a disc manufacturing apparatus using the mold.
The invention may be embodied in a disc-producing mold for forming a disc substrate having a circumferential direction by injection molding, comprising a cooling water ditch for distorting the disc substrate substantially evenly in the circumferential direction in forming the disc substrate. Accordingly, a disc substrate to be manufactured is prevented from having irregularities in thickness in the circumferential direction.
In the alternative the invention may be embodied in a disc-producing mold for forming a disc substrate having a circumferential direction by injection molding, comprising a cooling water ditch which revolves a plurality of turns with continuous crooks in the circumferential direction. This makes it possible to distort the mold evenly against injection pressure of molten resin, stopping a disc substrate from having irregularities in thickness in the circumferential direction.
According to a further feature of the invention, cooling water ditch is formed by a plurality of arcs of which radiuses are different from each other and connected to each other along the arcs. Thus, the cooling water ditch can easily be cut out and formed from the body of a mold.
According to a further feature of the invention, the cooling water ditch has a constant width part having an approximately constant width and connecting an inlet and an outlet of cooling water, a first convergent part connected to the inlet as well as located oppositely to the certain width part with the inlet being as a reference, and a second convergent end connected to the outlet as well as located oppositely to the certain width part with the outlet being as a reference. The existence of the convergent ends allows both of a cooling characteristic distribution of a mold given by the cooling water ditch and a strength distribution of the mold to be even in the vicinities of both ends of the cooling water ditch.
According to yet a further feature of the invention, the cooling water ditch has turns of three or more and a width of 3 mm or more. Setting the number of turns of the cooling water ditch to three or more can give the mold more even cooling and strength characteristics, whilst setting the width of the cooling water ditch to 3 mm or more can provide a necessary flow amount of cooling water into the cooling water ditch.
According to a further feature of the invention, the cooling water ditch is shaped into a configuration where half-arcs having mutually different two centers are alternately connected to each other, in which a distance A between the two centers is expressed by A=(G+L)/2, where G denotes a width of the cooling water ditch and L denotes an interval between adjoining ditch paths of the cooling water ditch. By this configuration, the cooling water ditch can be formed by mutually connecting half-arcs centered at different two points. Thus the cooling water ditch is formed easily.
In an embodiment of the invention, an area in which the cooling water ditch is formed is larger than an area occupied by the disc substrate to be manufactured. Accordingly, a disc substrate is formed within an area of which cooking characteristic and strength are uniform due to the formation of the cooling water ditch, resulting in that the disc substrate is manufactured with almost no thickness irregularities in its circumferential direction.
A disc substrate manufactured by the producing apparatus of the invention will have thickness irregularities diminished almost completely in its circumferential direction.